The present invention relates to a polybutene resin laminate, and more particularly, to polybutene resin laminate having excellent transparency, stretchability, flexibility, smoothness, heat sealability and self-adhesion, which may be stretched without undergoing generation of wrinkles and decrease in its transparency and fastening properties. The polybutene resin laminate of the present invention may be preferably used as a stretchable film for packaging foods.
Fresh and cooked foods are displayed and put on sale as a commodity after a so-called prepackaging to keep the food clean and free from dust deposition and contamination, as well as to maintain the freshness of the food. In the prepackaging of the fresh or cooked food, the food is either directly stretch-packaged with a transparent food-packing stretchable film, or stretch-packaged after placing the food on a plastic tray. The food-packaging stretchable film is stretched upon packaging in order to keep the item to be packaged under strictly sealed conditions. The food-packaging stretchable film is required to have a good transparency for visible packaging and for improving the commercial value of the item packaged with the film, and to have excellent air and vapor permeability, and aroma-retaining properties in order to retain the freshness and aroma of the fresh or cooked food, as well as good food-preserving ability at low temperatures. Furthermore, the food-packaging stretchable film is required to have good stretchability and elasticity to endure stretch-packaging with an automatic packaging machine, as well as satisfactory self-adhesion to prevent the film from being peeled off when the package is left for a long period after the packaging.
Films comprising a soft polyvinyl chloride (PVC) have been mainly employed for the food-packaging stretchable films which are required to have the previously disclosed properties.
The soft vinyl chloride resins, however, are liable to suffer from such problems as migration of the plasticizing agent contained in the resin into the food, which may result in hygienic problems, and from generation of hydrogen chloride gas upon incineration after the disposal. Therefore, use of the vinyl chloride film for the food-packaging stretchable film is problematic and development of a substitute for the polyvinyl chloride film is urgently required.
As substitutes for the soft vinyl chloride resin, there have been proposed laminates utilizing the properties of poly 1-butene, which has a high molecular weight among the polyolefins, and has rubbery properties similar to those of the polyvinyl chloride. Such laminates comprise an intermediate layer of poly 1-butene and surface layers of another polyolefin such as polyethylene, ethylene-vinyl acetate copolymer, and the like. For example, food-packaging stretchable films comprising three layers, that is polyolefin/poly 1-butene/polyolefin; ethylene-vinyl acetate copolymer/poly 1-butene/ethylene-vinyl acetate copolymer; and polyolefin/poly-1-butene/polyolefin are disclosed in Japanese Patent Publication No. 62(1987)-27981, Japanese Patent Application KOKAI Nos. 61(1986)-89040 and 1(1989)-166954.
In the above-described conventional laminates comprising at least three layers, the poly 1-butene used for the intermediate layer is either a homopolymer of 1-butene or a copolymer containing more that 85% by mole of 1-butene, resulting in a high crystallinity. Therefore, upon stretch packaging of the laminate film the poly 1-butene intermediate layer experiences oriented-crystallization causing irregular reflection of the light passing through the layer to result in cloudiness and decrease in transparency of the laminate. Also, the intermediate layer comprising a poly 1-butene homopolymer or a copolymer of high 1-butene content has a poor compatibility with the polyolefin surface layers. As a consequence, upon stretching of the laminate film, the surface layers are separated from the intermediate layer at the boundaries therebetween, and the layers are unevenly stretched to cause wrinkles. This results in poor appearance.